VB UWP No ConstructorArguments for ITypeSymbol Attributes

[karann-msft]

Affected Branch:
Affected Build: 15.1
Affected Product Language: VB only

Steps to reproduce:

1. Launch VS version 15.1
2. Create a UWP VB project, set platform target version to RS2
3. Open MainPage.xaml
4. Add inside the root Grid

Expected:
You get a min target version incompatible warning

Actual:
For VB project, you don't see this warning. This scenario works fine on C# and C++ UWP projects.

The root cause is we get the ContractVersionAttribute from a ITypeSymbol, and then get the apiContract name and version from the attributeData's ConstructorArguments. This works as expected for C# and C++ projects, but for VB, the ConstructorArguments are always empty.

This bug is blocking a new XAML feature for RS2 - warn about types and properties not available in the target platform minimum version supported by the project.

[AlekseyTs]

It appears the problem is in Private Function PEModuleSymbol.GetWellKnownType(type As WellKnownType, Optional careAboutAmbiguity As Boolean = True) As NamedTypeSymbol

The following code doesn't take into account the fact that currResult could be the same type as referencedAssemblyResult if the type was forwarded. Interestingly, C# compiler handles this properly in private NamedTypeSymbol PEModuleSymbol.GetTypeSymbolForWellKnownType(WellKnownType type)

                Dim currResult As NamedTypeSymbol = assembly.LookupTopLevelMetadataType(emittedName, digThroughForwardedTypes:=True)
                If IsAcceptableSystemTypeSymbol(currResult) Then
                    If referencedAssemblyResult Is Nothing Then
                        referencedAssemblyResult = currResult
                    Else
                        ' CONSIDER: setting result to null will result in a MissingMetadataTypeSymbol 
                        ' being returned.  Do we want to differentiate between no result and ambiguous
                        ' results?  There doesn't seem to be an existing error code for "duplicate well-
                        ' known type".
                        If (careAboutAmbiguity) Then
                            referencedAssemblyResult = Nothing
                        End If
                        Exit For
                    End If
                End If